Abstract: The dam-break process of clay soil dam under overtopping condition, related to multi-disciplines, is very complex. Although a large number of domestic and foreign physical model test results show that the headcut erosion caused by overtopping is the main breach process of most clay soil dams, the micro mechanism between water flow and dam body in the form of erosion is still not clear. The water flow plays a leading role in the development of dam break as it's the active force during overtopping failure. In the study, 3D numerical simulation research is carried out on the representative flow structure—the headcut flow, by using the RNG k-ε turbulence model and the volume of fluid (VOF) method. The flow structure, flow pattern and hydraulic characteristics of the dam headcut are analyzed in detail. Also, the distributions of shear stress and flow velocity in different working conditions are revealed. By using the research results, the analysis of the forces on dam body can be carried out from the point of view of hydrodynamics. The main way of all-level headcuts merged is inferred initially. The research results can help to make a clear understanding of the dam-break process.